1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an optical modulator having coplanar electrodes for controlling chirp. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for controlling a chirp in operating an optical modulator.
2. Discussion of the Related Art
An integrated optical modulator is of great interest in operating a fiber optical communication system in the range of 2.5 to 10 Gbps (Giga bits per second), and potentially 40 Gbps or above. A great deal of research has been carried out to quantify signal frequency broadening for different types of modulation. This effect is known as chirp. Chirp can cause loss of signal fidelity after propagating down the optical fiber due to a wavelength dispersion. In other words, chirp interacts with the dispersion profile of the transmission fiber to severely limit the distance over which error-free data propagation is possible.
There has been an effort to provide either fixed (zero or non-zero) or variable chirp by varying an arrangement of the system. Generation of chirp is system dependent, and it may include zero chirp, variable chirp, or chirp at a fixed non-zero value. Generally, modulators with variable chirp are more complicated, or require a more complex electrical driving scheme than fixed chirp modulators.
A conventional approach to control a chirp in external modulators has been to use an amplitude modulator and a phase modulator in series. The modulators are driven with adjustable voltages or pre-selected electrode lengths are used to obtain a desired value of chirp. A disadvantage of this approach is that the series configuration of the modulators generally leads to higher drive voltages due to a limited device length. Similar control of chirp can be obtained in a single Mach Zehnder amplitude modulator in which the arms of the interferometer are driven with independent drive voltages with adjustable amplitude and phase. A drive voltage or a voltage required to turn the modulator from an off-state to an on-state is one of the important features for external modulators. By minimizing the voltage, the drive power required to operate the modulator can be minimized.
For Mach Zehnder amplitude modulators biased at their quadrature or linearly operating point, zero chirp can be obtained by driving the arms of the interferometer in a symmetrical fashion, so that the light in each arm receives equal and opposite phase shifts. One way to achieve this feature is to apply equal and oppositely directed electric fields to each arm of the interferometer, while ensuring that the electro-optic overlap integrals are the same for each arm. Fixed non-zero chirp may be created by varying a magnitude of the field, a magnitude of the overlap integral, or both in one of the interferometer arms. Nonetheless, it is difficult to achieve zero or fixed non-zero chirp parameter in the conventional way.
Accordingly, the present invention is directed to an optical modulator having coplanar electrodes for controlling a chirp that substantially obviates one or more problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide an optical modulator controlling a chirp in operating an optical communication system.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an optical modulator includes a substrate having an electrooptic effect, an optical waveguide having first and second cascading portions in the substrate, and transmitting an optical field, a first coplanar strip electrode having a first part over the first cascading portion and second and third parts extending beyond the first cascading portion, wherein the first part is approximately perpendicular to the second and third parts and has a width substantially the same as the first cascading portion, a second coplanar strip electrode having a first part over the second cascading portion and second and third parts to extend beyond the second cascading portion, and the first part being approximately perpendicular to the second and third parts, and a voltage source supplying a voltage to the first coplanar strip electrode, wherein the second coplanar strip electrode is grounded and is symmetrical to the first coplanar strip electrode.
In another aspect of the present invention, an optical modulator includes a substrate having an electrooptic effect, an optical waveguide having first and second cascading portions in the substrate, and transmitting an optical field, a first coplanar waveguide electrode having first, second and third parts, wherein the first part is approximately perpendicular to the second and third parts and does not overlap the first and second cascading portions, a second coplanar waveguide electrode having a first part over the first cascading portion and second and third parts extending beyond the first cascading portion, wherein the first part is approximately perpendicular to the second and third parts, a third coplanar waveguide electrode having a shape symmetric to the first coplanar waveguide electrode and separated apart from the second coplanar waveguide electrode by a distance the same as the distance between the first and second coplanar waveguide electrodes, and a voltage source supplying a voltage to the second coplanar waveguide electrode, wherein the first and third coplanar waveguide electrodes are grounded, so that electrooptic overlap integrals of each cascading portion of the optical waveguide are different, thereby generating a fixed non-zero amount of a modulation chirp parameter.
In another aspect of the present invention, an optical modulator includes a substrate having an electrooptic effect, an optical waveguide having first and second cascading portions in the substrate, and transmitting an optical field, a first coplanar waveguide electrode having first, second and third parts, wherein the first part is approximately perpendicular to the second and third parts and does not overlap the first and second cascading portions, and has a width substantially greater that the first and second cascading portions, a second coplanar waveguide electrode having a first part over the first cascading portion and second and third parts extending beyond the first cascading portion, wherein the first part is approximately perpendicular to the second and third parts, a third coplanar waveguide electrode having a shape symmetric to the first coplanar waveguide electrode and separated apart from the second coplanar waveguide electrode by a distance the same as the distance between the first and second coplanar waveguide electrodes, wherein the first parts of the first and third coplanar waveguide electrodes have a width substantially greater than that of the second coplanar waveguide electrode, and a voltage source supplying a voltage to the second coplanar waveguide electrode, wherein the first and third coplanar waveguide electrodes are grounded, so that electrooptic overlap integrals of each cascading portion of the optical waveguide is different, thereby generating a fixed non-zero amount of a modulation chirp parameter.
In a further aspect of the present invention, an optical modulator includes a substrate having an electrooptic effect, an optical waveguide having first and second cascading portions in the substrate, and transmitting an optical field, a first coplanar waveguide electrode having a first part over the second cascading portion and second and third parts extending beyond the second cascading portion, wherein the first part is approximately perpendicular to the second and third parts, a second coplanar waveguide electrode having a first part over the first cascading portion and second and third parts extending beyond the first cascading portion, wherein the first part is approximately perpendicular to the second and third parts, a third coplanar waveguide electrode having a shape symmetric to the first coplanar waveguide electrode and separated apart from the second coplanar waveguide electrode by a distance the same as the distance between the first and second coplanar waveguide electrodes, a fourth coplanar waveguide electrode having a shape symmetric to the first coplanar waveguide electrode, and having a first part over the second cascading portion and being in contact with the first part of the first coplanar waveguide electrode, a fifth coplanar waveguide electrode having a shape symmetric to the second coplanar waveguide electrode and separated apart from the fourth coplanar waveguide electrode by a distance the same as the distance between the first and second coplanar waveguide electrodes, a sixth coplanar waveguide electrode having a shape symmetric to the third coplanar waveguide electrode and separated apart from the fifth coplanar waveguide electrode by a distance the same as the distance between the second and third coplanar waveguide electrodes, and a voltage source supplying a voltage to the second and fifth coplanar waveguide electrodes, wherein the first, third, fourth, and sixth coplanar waveguide electrodes are grounded.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.